Slab on Ground Compliance for Australian Homes: NCC Part H1 and AS 2870

What it is

Slab on ground is the dominant footing system for Class 1 detached houses, terraces and townhouses across Australia. The compliance framework is set by NCC 2022 Volume Two Part H1 Structure, which calls up AS 2870-2011 Residential Slabs and Footings as the primary Deemed-to-Satisfy standard.

The job of the slab is structural and serviceability. It transfers building load to the soil and limits movement so that walls, cornices and brickwork do not crack beyond AS 2870 Appendix C damage categories. On reactive clays the soil itself moves with moisture changes, and the slab has to ride that movement without distress.

Site classification under AS 2870

Every site gets a class. The geotechnical engineer reports it after a borehole or test pit investigation. The classes are A (sand and rock, little to no movement), S (slightly reactive clay), M (moderately reactive), H1 and H2 (highly reactive), E (extremely reactive), and P (problem sites such as fill, soft soils, mine subsidence or trees).

The class determines slab type. A and S sites can use a stiffened raft to AS 2870 with modest edge beams. M sites step up edge beam depths and add internal beams. H1 and H2 sites typically need a waffle pod raft or a deep stiffened raft with extra reinforcement. E sites usually push the design to a piled or screw-pile suspended slab. P sites require a specific engineer-designed solution outside the standard recipes.

What ends up on the slab plan

A compliant slab plan shows the site class, the design wind speed, the slab thickness, the edge beam dimensions, the internal beam grid, the top and bottom reinforcement schedule (mesh grade and bar overlay), the vapour barrier specification (AS 2870 Section 5.3.3 requires 0.2 mm polyethylene under habitable areas), and any articulation joints.

Where slabs fail an inspection

Defect notices on residential slabs cluster around the same handful of items.

Reinforcement does not match the design

The mesh shown on the plan is SL92 but SL82 is delivered. Bar chairs are missing or crushed under foot traffic before the pour, leaving mesh on the ground rather than at the design cover. Edge beam trench steel has the wrong number of N16 bars or is too close to the formwork.

Site class mismatch

The geotech report calls the site M but the slab is poured to a Class S detail. This happens when a footing system gets selected before the soil report is finalised, then never gets reissued. The certifier signs the building permit on the original drawing.

Vapour barrier issues

Punctures around penetrations, taped joints that have come undone, or sand blinding placed over a torn membrane. Each of these breaks the moisture barrier and creates a path for slab-edge dampness and efflorescence.

Articulation joints missed on masonry

AS 2870 requires articulation in masonry above reactive sites at spacings tied to the site class. The slab can be compliant and the brickwork above it still fail because the bricklayer ran the wall through without a control joint.

Inspection hold points

For a TradeLens review of a slab pour, the hold points are pre-pour reinforcement inspection, pre-pour formwork and setout check, and post-pour curing. The pre-pour inspection is the load-bearing one. The auditor checks bar size and number against the engineer drawing, cover to the underside (40 mm to ground, 30 mm internal where there is a vapour barrier), bar laps (typically 500 mm for N12), edge beam depth, and vapour barrier continuity.

Cube tests and core results sit alongside the inspection. AS 3600 sets the concrete grade and minimum strength at 28 days. N20 is the minimum for residential slab on ground in most cases, with N25 or higher on aggressive soils or where the engineer specifies.

Why an auditor cares

Slab defects are expensive to fix and almost impossible to hide. A cracked slab on a reactive site is a 20-year claim risk for the builder. The audit pattern is the same. Was the site classified by a competent geotech. Did the slab design match the site class. Was the reinforcement inspected before the pour. Was concrete strength confirmed. Was articulation built into the masonry above.

Each of those questions points to a clause in Part H1 or AS 2870. A builder who pours by the plan and keeps the pre-pour photos rarely loses an audit on slab work.